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11 Intermolecular Forces Solutions to Exercises (b) (c) These two molecules have the same molecular formula and molecular weight (C₃H₇Cl, molecular weight = 78.5 amu), so the shapes of the molecules determine which has the stronger dispersion forces. According to Figure 11.6, the cylindrical (not branched) molecule will have stronger dispersion forces. 11.23 Both hydrocarbons experience dispersion forces. Rod-like butane molecules can contact each other over the length of the molecule, while spherical 2-methylpropane molecules can only touch tangentially. The larger contact surface of butane facilitates stronger forces and produces a higher boiling point. 11.24 Both molecules experience hydrogen bonding through their -OH groups and dispersion forces between their hydrocarbon portions. The position of the -OH group in isopropyl alcohol shields it somewhat from approach by other molecules and slightly decreases the extent of hydrogen bonding. Also, isopropyl alcohol is less rod-like (it has a shorter chain) than propyl alcohol, so dispersion forces are weaker. Since hydrogen bonding and dispersion forces are weaker in isopropyl alcohol, it has the lower boiling point. 11.25 (a) A molecule must contain H atoms bound to either N, o or F atoms in order to participate in hydrogen bonding with like molecules. (b) and have N-H and O-H bonds, respectively; they will form hydrogen bonds with other molecules of the same kind. (CH₃F has C-F and C-H bonds, but no H-F bonds.) 11.26 (a) HF has the higher boiling point because hydrogen bonding is stronger than dipole-dipole forces. (b) CHBr₃ has the higher boiling point because it has the higher molar mass, which leads to greater polarizability and stronger dispersion forces. (c) ICI has the higher boiling point because it is a polar molecule. For molecules with similar structures and molar masses, dipole-dipole forces are stronger than dispersion forces. 11.27 (a) Replacing a hydroxyl hydrogen with a CH₃ group eliminates hydrogen bonding in that part of the molecule. This reduces the strength of intermolecular forces and leads to a (much) lower boiling point. (b) is a larger, more polarizable molecule with stronger London-dispersion forces and thus a higher boiling point. 11.28 (a) C₃H₈, dispersion; C₄H₁₀, dispersion. C₄H₁₀ has the higher boiling point due to greater molar mass and similar strength of forces. (b) CH₃CH₂OCH₂CH₃, dipole-dipole, and dispersion; hydrogen bonding, dipole-dipole, and dispersion. has the higher boiling point due to the influence of hydrogen bonding. (c) dipole-dipole and dispersion; SO₃, dispersion. This is a tough call; SO₂ has dipole-dipole forces, but SO₃ has a greater molecular weight. The relative strength of dispersion and dipole-dipole forces depends on the mass and shape 314